Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 AM EST Tue Nov 19 2024 Valid 12Z Fri Nov 22 2024 - 12Z Tue Nov 26 2024 ***Strong, long-duration atmospheric river will take aim at northern California/southwestern Oregon through late week*** ...General Overview... A long duration atmospheric river is forecast to affect the West Coast into later this week as upper and surface lows/frontal systems direct a strong moisture plume into northern California and southwestern Oregon, where several inches of rain are likely to cause river rising and some flooding concerns. Meanwhile a deep upper low will drift slowly from the Great Lakes to the Northeast U.S., leading to cooler temperatures and a cold rain/higher elevation snow for those regions. In between, an upper ridge is likely to traverse the interior West and central U.S. before the upper pattern becomes a bit more zonal into early next week. ...Guidance/Predictability Assessment... The models have come into good agreement with the large upper low and surface low pressure system across the Eastern U.S. late in the week, and a general deterministic model compromise works well as a starting point in the forecast process. The guidance has also improved with the expected quasi-zonal flow evolution across much of the central U.S., and are showing more signs of a deepening trough over the Rockies by early next week, which could herald the arrival of an arctic airmass over the far northern Plains. The ensemble means were gradually increased to about half by next Tuesday amid growing model differences at the mesoscale level, while still maintaining some of the deterministic GFS and ECMWF solutions. ...Weather/Hazards Highlights... Confidence is high for a prolonged atmospheric river extending into Friday across portions of northwestern California and into southwestern Oregon. Enhanced moisture flux ahead of a slow moving cold front will result in enhanced orographic lift and likely produce some additional 2 to 4 inch rainfall totals by early Saturday. A Slight Risk area is in place for Day 4 across these areas, including the coastal ranges/northern Sierra and extending to the coast. There will likely be some abatement going into Saturday with the front slowly making its way inland across the region. Snow levels will initially be high, but should fall going into the weekend, including much of the Sierra range where heavy snow is becoming more likely. However, there is a Marginal Risk area for the lower elevations of the western Sierra where upslope flow may result in some rainfall totals of 1-2 inches on Day 5. Impactful weather is also expected for the Northeast U.S. to close out the work week. This will be the result of a large upper low positioned over the region, in combination with a potentially sub-990 mb surface low over New England. Cold air aloft will be sufficient to support some accumulating early season snow for the higher terrain near the Great Lakes and interior Northeast, and especially for the central Appalachians where some totals in excess of six inches are expected. There will also be some lake effect rain and snow in the wake of this storm system. The strong surface low will result in windy conditions and periods of heavy rain for much of New England through Friday afternoon. Temperatures will turn noticeably colder across the Eastern U.S. in the wake of a strong cold front and the upper low moving in overhead by Thursday, with highs expected to be about 5 to 15 degrees below average to close out the work week. Given widespread cloud cover, overnight lows will be closer to average with less of a diurnal range for many areas. A warming trend is forecast across the Southern Plains going into the weekend as an upper level ridge builds in and warmer flow from the Gulf of Mexico, with highs reaching into the 80s for the southern half of Texas. Signs of an arctic airmass may start to be noticed for parts of Montana and North Dakota by next Monday, with overnight lows dropping into the 0s and low 10s. Even colder weather may arrive in time for Thanksgiving for those areas. Hamrick Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation forecast (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw